DESCRIPTION (adapted from the applicant's description): The overall hypothesis of this proposal is that both basal and stimulated L-type Ca currents (ICaL) in adult atrial cells are regulated synergistically by both cGMP and cAMP. This hypothesis builds on the principal investigator's novel observation that adult atrial cells retain their neonatal responsiveness to cGMP in contrast to adult ventricular cells, which do not. The proposal tests five specific predictions of the overall hypothesis. (1) Basal atrial ICa,L is modulated synergistically by cGMP and cAMP. Single rabbit atrial cells will be exposed to cGMP and/or cAMP via a perfusible patch pipette. (2) Stimulated atrial ICa,L is modulated synergistically by cGMP and cAMP. (3) The mechanism of PKG stimulated atrial ICa,L involves increased L-type Ca channel availability. (4) Site-specific differences in PKG activity correlate with differences in isoform expression. Expression of PKG isoforms (type I alpha & beta & type II) will be determined by Western immunoblotting with specific antibodies and by mRNA expression. (5) The maintained responsiveness of adult atrial ICa,L to cGMP compared to ventricular cells is a consequence of 2 phosphorylation sites, which as phosphorylated independently by PKG and PKA.